Differential device

ABSTRACT

In a differential device, paired side gears include shaft portions and intermediate wall portions formed in a flat plate shape intersecting an axis of the output shafts, the intermediate wall portions integrally connecting the shaft portions to toothing portions of the side gears separated outward from the shaft portions in a radial direction of an input member, respectively. Each of the intermediate wall portions is formed with a width in the radial direction which is longer than a maximum diameter of a pinion. A cover portion includes a boss portion and a side wall portion connected to the boss portion and having an outer side surface as a surface orthogonal to the axis of the output shafts. A back surface of at least one of the intermediate wall portion and the toothing portion is supported on an inner side surface of the side wall portion.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an improvement of a differential devicewhich distributively transmits rotational force of an input member to apair of mutually-independent output shafts, the input member retaining apinion support portion that supports a pinion and being rotatabletogether with the pinion support portion. Particularly, the presentinvention relates to an improvement of the differential device thatincludes: a pair of side gears each having an outer peripheral portionwhich includes an annular toothing portion in mesh with the pinion, andconnected to the pair of output shafts, respectively; and a coverportion covering an outside of at least one side gear of the side gearsand rotating integrally with the input member.

Description of the Related Art

Conventionally, such a differential device is widely known as disclosedin, for example, Japanese Patent No. 4803871 and Japanese PatentApplication KOKAI Publication No. 2002-364728.

The design of a differential device has been desired to be improved suchthat each of the side gears can have a sufficiently large diameter tohave a sufficiently larger number of teeth than the number of teeth ofeach pinion, and that the side gears and the cover portions arrangedoutside of the side gears can be reduced in thickness in the axialdirection of the output shafts. However, the conventional differentialdevice has not been improved sufficiently in the aforementioned points,and the differential device itself still has a structural formrelatively wide in the axial direction of the output shafts. This maycause undesirable things such as: difficulty in incorporating thedifferential device in a transmission system under many layoutrestrictions around the differential device; increase in the size of thetransmission system as a whole due to the incorporation of thedifferential device therein; and the like.

SUMMARY OF THE INVENTION

The present invention has been made with the foregoing situation takeninto consideration. An object of the present invention is to provide thedifferential device which can solve the above-discussed problems.

In order to achieve the object, a differential device according to thepresent invention, distributively transmits rotational force of an inputmember to a pair of mutually-independent output shafts, the input memberretaining a pinion support portion that supports a pinion and beingrotatable together with the pinion support portion, wherein thedifferential device comprises: a pair of side gears each having anannular toothing portion in mesh with the pinion in an outer peripheralportion and connected to the pair of output shafts, respectively; and acover portion covering an outside of at least one side gear of the sidegears and rotating integrally with the input member, wherein the pair ofside gears include shaft portions connected to the pair of outputshafts, respectively, and intermediate wall portions formed in a flatplate shape intersecting an axis of the output shafts, the intermediatewall portions integrally connecting the shaft portions to the toothingportions separated outward from the shaft portions in a radial directionof the input member, respectively, each of the intermediate wall portionis formed with a width in the radial direction which is longer than amaximum diameter of the pinion, the cover portion includes a bossportion concentrically surrounding the corresponding shaft portion, anda plate-shaped side wall portion connected to the boss portion andhaving an outer side surface as a surface orthogonal to the axis of theoutput shafts, and a back surface of at least one of the intermediatewall portion and the toothing portion is rotatably supported on an innerside surface of the side wall portion. (This is a characteristic of thepresent invention.)

According to the first characteristic, the side gears include: the shaftportions connected to the output shafts, respectively; and theintermediate wall portions each formed in the flat plate shapeintersecting the axis of the output shafts, and integrally connectingthe shaft portions to the toothing portions of the side gears separatedoutward from the shaft portions in the radial direction of the inputmember, respectively. In addition, each intermediate wall portion isformed with the width in the radial direction which is longer than themaximum diameter of the pinion. Thus, the diameter of each side gear canbe sufficiently larger than that of the pinion such that the number ofteeth of the side gear can be set sufficiently larger than the number ofteeth of the pinion. This makes it possible to reduce load burden to thepinion support portion in transmission of torque from the pinion to theside gears, and thus to achieve reduction in the effective diameter ofthe pinion support portion and accordingly reduction in the width of thepinion in the axial direction of the output shafts. Furthermore, sincereaction force applied to each side gear decreases with the reduction inthe load to burden to the pinion support portion as described above, andsince the back surface of the intermediate wall portion or the toothingportion of the side gear is supported on the side wall portion of thecover portion, the thickness of the intermediate wall portion of theside gear can be sufficiently reduced while securing the supportrigidity to the side gear. Moreover, since the side wall portion of thecover portion is formed in the plate shape in which the outer sidesurface of the side wall portion is the surface orthogonal to the axisof the output shafts, the thickness of the side wall portion of thecover portion itself can be reduced as well. Thus, the differentialdevice of the present invention as a whole can be sufficiently reducedin width in the axial direction of the output shafts while securingstrength (for example, static torsion load strength) and the maximumamount of torque transmission at approximately the same levels as theconventional differential device. Accordingly, the differential devicecan be easily incorporated in a transmission system, which is under manylayout restrictions around the differential device, with great latitudeand no specific difficulties, and is therefore advantageous in reducingthe size of the transmission system.

In the differential device according to the present invention,preferably, the intermediate wall portion of the at least one side gearis formed with a maximum thickness in an axial direction of the outputshafts which is smaller than an effective diameter of the pinion supportportion. (This is a second characteristic of the present invention.)

According to the present invention, the thickness of the intermediatewall portion of the side gear can be reduced as described above. Inaddition, according to the second characteristic of the presentinvention, the maximum thickness of the intermediate wall portion of theside gear can be formed much smaller than the effective diameter of thepinion support portion which can be reduced in diameter. Thus, thedifferential device can be further reduced in width in the axialdirection.

In the differential device according to the present invention,preferably, the input member has an input toothing portion whichreceives the rotational force from a power source in an outer peripheralportion, and the side wall portion of the cover portion is disposedwithin a width of the input member in the axial direction of the outputshafts. (This is a third characteristic of the present invention.)

According to the third characteristic, the side wall portion of thecover portion is disposed within the width of the input member, whichhas the input toothing portion in the outer peripheral portion, in theaxial direction of the output shafts. Thus, the differential device canachieve a further reduction in the width in the axial direction of theoutput shafts with the side wall portion of the cover portion kept fromprotruding outward beyond an end surface of the input member in theaxial direction of the output shafts.

In the differential device according to the present invention,preferably, a relationship expressed with d3≧3.74·d2+20 is satisfied,where d2 denotes an effective diameter of the pinion support portion andd3 denotes a load point length of the pinion. (This is a fourthcharacteristic of the present invention.)

According to the fourth characteristic, the relationship expressed withd3≧3.74·d2+20is satisfied, where d2 denotes the effective diameter of the pinionsupport portion and d3 denotes the load point length of the pinion.Thus, the load point length of the pinion can be sufficiently lengthenedand the differential device can be sufficiently reduced in width in theaxial direction of the output shafts, while securing static torsion loadstrength at a level approximately the same as or greater than that ofthe conventional differential device.

In the differential device according to the present invention,preferably, the pinion and the toothing portion of each of the sidegears are bevel gears, and relationships expressed with Z1/Z2≧2, andPCD≧6.17·(Z1/Z2 )+20 are satisfied, where PCD denotes a pitch conedistance of the pinion, Z2 denotes the number of teeth of the pinion andZ1 denotes the number of teeth of the side gear. (This is a fifthcharacteristic of the present invention.)

According to the fifth characteristic, the relationships expressed withZ1/Z2≧2, andPCD≧6.17·(Z1/Z2)+20are satisfied, where PCD denotes the pitch cone distance of the pinionformed of a bevel gear, Z2 denotes the number of teeth of the pinion andZ1 denotes the number of teeth of each side gear. This enables thedifferential device to be sufficiently reduced in width in the axialdirection of the output shafts, while securing the maximum amount oftorque transmission at a level approximately the same as or greater thanthat of the conventional differential device.

Note that, in the present invention, “the effective diameter of thepinion support portion” refers to an outer diameter d2 of a shaft as thepinion support portion (in the present embodiment, a pinion shaft PS ora support shaft portion PS′) which is formed separately from orintegrally with the pinion so as to support the pinion and so as to beattached to the input member.

In addition, in order to achieve the object, a differential deviceaccording to the present invention, comprises: an input member intowhich driving force is input; a differential gear support portionsupported in the input member; a differential gear supported on thedifferential gear support portion; and a pair of output gears in meshwith the differential gear and relatively rotatable to the differentialgear, wherein

${d\;{2/{PCD}}} \leq {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}$is satisfied, and

Z1/Z2>2 is satisfied, where Z1, Z2 , d2 and PCD denote the number ofteeth of each of the output gears, the number of teeth of thedifferential gear, a diameter of the differential gear support portionand a pitch cone distance, respectively. (This is a sixth characteristicof the present invention.)

According to the sixth characteristic, the differential device as awhole can be sufficiently reduced in width in the axial direction of theoutput shafts while securing the strength (for example, the statictorsion load strength) and the maximum amount of torque transmission atapproximately the same levels as the conventional differential device.Accordingly, the differential device can be easily incorporated in atransmission system, which is under many layout restrictions around thedifferential device, with great latitude and no specific difficulties,and is therefore advantageous in reducing the size of the transmissionsystem.

In the differential device according to the present invention,preferably, Z1/Z2≧4 is satisfied. (This is a seventh characteristic ofthe present invention.)

In the differential device according to the present invention,preferably, Z1/Z2≧5.8 is satisfied. (This is an eighth characteristic ofthe present invention.)

According to the seventh and eighth characteristics, the differentialdevice can be more sufficiently reduced in width in the axial directionof the output shafts while securing the strength (for example, thestatic torsion load strength) and the maximum amount of torquetransmission at approximately the same levels as the conventionaldifferential device.

The above and other objects, characteristics and advantages of thepresent invention will be clear from detailed descriptions of thepreferred embodiments which will be provided below while referring tothe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a differential device and itsvicinity of an embodiment of the present invention (a sectional viewtaken along a 1-1 line in FIG. 2).

FIG. 2 is a partially cutaway side view of the differential device (asectional view taken along a 2-2 line in FIG. 1).

FIGS. 3A and 3B show modified embodiments of an input member of thedifferential device, FIG. 3A shows the input member having an innerperipheral surface which is formed by joining two arcs with the samediameter, and FIG. 3B shows the input member having an inner peripheralsurface with an elliptic shape.

FIG. 4 is a side view showing a modified embodiment of a cover portionof the differential device, and corresponding to FIG. 2.

FIGS. 5A to 5D show other modified embodiments of the differentialdevice, FIG. 5A shows a modified embodiment in which a washer retaininggroove is provided to the cover portion, FIG. 5B shows a modifiedembodiment in which the washer retaining groove is provided to a backsurface of a toothing portion of a side gear, FIG. 5C shows a modifiedembodiment in which the washer retaining groove is omitted bypositioning and retaining a washer by using an overhanging portion of aninner side surface of the cover portion, and FIG. 5D shows a modifiedembodiment in which an intermediate wall portion of the side gear isdisposed closer to a pinion shaft side by decreasing a diameter of anintermediate portion of the pinion shaft.

FIGS. 6A to 6D are step explanatory views showing an example of a stepfor assembling the differential device.

FIG. 7 is a partial sectional view showing a modified embodiment of apinion support portion in the differential device, and corresponding toFIG. 1.

FIGS. 8A to 8C are graphs for comparing a setting example of thedifferential device of the embodiment and a setting example of aconventional differential device, FIG. 8A shows a relationship between apinion shaft diameter and a load point length of a pinion, FIG. 8B showsa relationship between a gear ratio of the side gear and the pinion anda pitch cone distance of the pinion, and FIG. 8C shows a relationshipbetween the gear ratio and a width in an axial direction of thedifferential device.

FIG. 9 is a partial sectional view showing a modified embodiment of theside gear and the cover portion in the differential device, andcorresponding to FIG. 1.

FIG. 10 is a longitudinal cross-sectional view showing an example of theconventional differential device.

FIG. 11 is a graph showing a relationship of gear strength change rateswith a number-of-teeth ratio where the number of teeth of the pinion isset at 10.

FIG. 12 is a graph showing a relationship of the gear strength changerates with a pitch cone distance change rate.

FIG. 13 is a graph showing a relationship of the pitch cone distancechange rate with the number-of-teeth ratio for keeping 100% of the gearstrength where the number of teeth of the pinion is set at 10.

FIG. 14 is a graph showing a relationship between a shaft diameter/pitchcone distance ratio and the number-of-teeth ratio where the number ofteeth of the pinion is set at 10.

FIG. 15 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 6.

FIG. 16 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 12.

FIG. 17 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described based on thedrawings.

To begin with, in FIGS. 1 and 2, a differential device D drives a pairof left and right axles while allowing differential rotation thereof, bydistributively transmitting rotational driving force, which istransmitted from an engine (not illustrated) mounted on an automobile,to a pair of left and right output shafts A continuous to the left andright axles. The differential device D is housed and supported, forexample, inside a transmission case 1 fixed to the center of a rearportion of a vehicle body.

The differential device D includes: multiple pinions P; a pinion shaftPS as a pinion support portion which rotatably supports the pinions P;an input member I having a short cylindrical shape and supporting thepinion shaft PS so as to be capable of rotating together with the pinionshaft PS; a pair of left and right side gears S in mesh with the pinionsP from both the left and right sides, and connected respectively to thepair of left and right output shafts A; and a pair of left and rightcover portions C, C′ covering outer sides of the respective side gearsS, and rotating integrally with the input member I. A differential caseDC is formed from the input member I and the cover portions C, C′.

Incidentally, the embodiment shows the differential device D whichincludes two pinions P, and whose pinion shaft PS as the pinion supportportion is formed in a linear rod shape extending along one diameterline of the input member I with the two pinions P respectively supportedby both end portions of the pinion shaft PS. Instead, the differentialdevice D may include three or more pinions P. In this case, the pinionshaft PS is formed in a shape of crossing rods such that rods extendradially from a rotation axis L of the input member I in three or moredirections corresponding to the three or more pinions P (for example, ina shape of a cross when the differential device D includes four pinionsP), and tip end portions of the extending rods support the pinions P,respectively.

In addition, the pinions P may be fitted to the pinion shaft PS directlyas shown in the illustrated example, or with bearing means (notillustrated), such as a bearing bush and the like, interposed betweenthe pinion shaft PS and each pinion P. Furthermore, the pinion shaft PSmay be formed in a shape of a shaft whose diameter is equal throughoutits whole length as shown in the illustrated example, or formed in ashape of a stepped shaft.

The differential case DC is rotatably supported by the transmission case1 with left and right bearings 2 interposed therebetween. Moreover,through-holes 1 a through which to insert the output shafts A are formedin the transmission case 1. Annular seal members 3 for sealinginterstices between inner peripheries of the through-holes 1 a and outerperipheries of the output shafts A are interposed between the innerperipheries and the outer peripheries. Furthermore, an oil pan (notillustrated) facing an inner space of the transmission case 1 andreserving a predetermined amount of lubricant oil is provided in abottom portion of the transmission case 1. Mechanical interlockingsections existing inside and outside the differential case DC can belubricated with the lubricant oil which is scattered around thedifferential device D by rotation of the differential case DC and theother rotary members.

An input toothing portion Ig as a final driven gear is provided in anouter peripheral portion of the input member I. This input toothingportion Ig is in mesh with a drive gear (not illustrated) which isrotationally driven by power of the engine. Incidentally, in theembodiment, the input toothing portion Ig is directly formed in an outerperipheral surface of the input member I over a full lateral width ofthe input member I (i.e., an overall axial width of the input member I).Instead, however, the input toothing portion Ig may be formed to havethe width smaller than that of the input member I. Otherwise, the inputtoothing portion Ig may be formed separately from the input member I,and thereafter fixed to the outer peripheral portion of the input memberI.

Meanwhile, in the embodiment, the pinions P and the side gears S areeach formed as a bevel gear. In addition, each pinion P as a whole andeach side gear S as a whole, including their toothing portions, areformed by plastic working such as forging and the like. For thesereasons, their toothing portions with an arbitrary gear ratio can beprecisely formed without restriction in machining work in the case wherethe toothing portions of the pinions P and the side gears S are formedby cutting work. Incidentally, other types of gears may be used insteadof the bevel gear. For example, a face gear may be used for the sidegears S, while a spur gear or a helical gear may be used for the pinionsP.

In addition, the pair of side gears S each include: a cylindrical shaftportion Sj to which an inner end portion of the corresponding one of thepair of output shafts A is connected by being spline-fitted as at 4; anannular toothing portion Sg situated at a position separated outwardfrom the shaft portion Sj in a radial direction of the input member I,and being in mesh with the corresponding pinion P; and an intermediatewall portion Sw formed in a flat ring plate shape orthogonal to the axisL of the corresponding output shaft A, and integrally joining the shaftportion Sj and the toothing portion Sg.

The intermediate wall portion Sw is formed with its width t1 in theradial direction larger than a maximum diameter d1 of the pinion P, andwith its maximum thickness t2 in an axial direction of the output shaftA smaller than an effective diameter d2 of the pinion shaft PS (see FIG.1). Thereby, as described later, a diameter of the side gear S can bemade large enough to set the number Z1 of teeth of the side gear Ssufficiently larger than the number Z2 of teeth of the pinions P, andthe side gear S can be sufficiently thinned in the axial direction ofthe output shaft A.

Moreover, the cover portion C, which is one of the pair of coverportions C, C′, is formed separately from the input member I, and isdetachably connected to the input member I using bolts b. The connectingmethod may use various connecting means other than screw means. Examplesof the various connecting means include welding means and swaging means.Meanwhile, the other cover portion C′ is formed integral with the inputmember I. Incidentally, like the one cover portion C, the other coverportion C′ may be formed separately from the input member I, andconnected to the input member I using bolts b or other connecting means.

Besides, each of the cover portions C, C′ includes: a cylindrical bossportion Cb which concentrically surrounds the shaft portion Sj of theside gear S and in which the shaft portion Sj is rotatably fitted andsupported; and a plate-shaped side wall portion Cs having an outer sidesurface which is a flat surface orthogonal to the rotation axis L of theinput member I, the side wall portion Cs integrally connected to aninner end in an axial direction of the boss portion Cb. The side wallportions Cs of the cover portions C, C′ are disposed within the width ofthe input member I (accordingly, the input toothing portion Ig) in theaxial direction of the output shaft A. This inhibits the side wallportions Cs of the cover portions C, C′ from protruding outward from anend surface of the input member I in the direction of the rotationalaxis, and is thus advantageous in making a width of the differentialdevice D narrower in the axial direction of the output shaft.

In addition, by inner side surfaces of the side wall portions Cs of thecover portions C, C′, back surfaces of at least one of the intermediatewall portions Sw and the toothing portions Sg of the side gears S arerotatably supported via washers W, respectively. Incidentally, suchwashers W may be omitted so that by the inner side surfaces of the sidewall portions Cs, the back surfaces of at least one of the intermediatewall portions Sw and the toothing portions Sg of the side gears S arerotatably and directly supported, respectively. Furthermore, the shaftportions Sj of the side gears S may be supported by the boss portions Cbof the cover portions C, C′ via bearings, respectively.

Meanwhile, the input member I surrounds an entire periphery of each sidegear S with an inner peripheral surface Ii of the input member I beingclose to an outer peripheral portions of the side gear S. Furthermore,as also shown in FIG. 2, among the inner peripheral surface Ii of theinput member I, particularly a predetermined inner peripheral part Iiasituated around each pinion P is formed in a recessed shape so as to beseparated farther from the rotation axis L of the input member I thanthe other inner peripheral parts, and thus forms an oil reservoir. Forthis reason, the oil reservoir can effectively collect and reserve thelubricant oil due to centrifugal force produced by rotation of the inputmember I, and a large amount of lubricant oil collected there can beefficiently supplied to the corresponding pinion P and its vicinity.Accordingly, even under severe driving conditions and the like, such ashigh-speed rotation of the pinions P, the lubricant oil can besufficiently supplied to sliding portions of the pinions P and meshingportions of the pinions P and the side gears S. This is effective toprevent seizure in the sliding portions and the mesh portions.

In the differential device D of the embodiment in particular, asdescribed above, the diameter of the side gears S (accordingly, thedifferential case DC) can be made large enough and the larger amount oflubricant oil can be efficiently collected into the predetermined innerperipheral parts Iia (the oil reservoirs) in the input member I usingthe larger centrifugal force. For this reason, even though the largerdiameter of the side gears S makes the pinions P rotate at faster speed,an obvious seizure preventing effect can be obtained.

In the embodiment, the predetermined inner peripheral parts Iia servingas the oil reservoirs are each formed in a shape of an arc, whosecurvature is larger than that of the other inner peripheral parts on across-section orthogonal to the rotation axis L of the input member I.In addition, in the embodiment (FIG. 2), the predetermined innerperipheral parts Iia are each formed as a first arc with a relativelysmall diameter, whose center O′ is offset from the rotation axis L ofthe input member I toward the corresponding pinion P on thecross-section; and the other inner peripheral parts are each formed as asecond arc with a diameter larger than that of the first arc, whosecenter O is situated on the rotation axis L of the input member I on thecross-section. Thereby, even when the predetermined inner peripheralparts Iia (the oil reservoirs) are set in relatively narrow areas in aperipheral direction, the predetermined inner peripheral parts Iia canbe formed deeply enough on sides separating from the rotation axis L ofthe input member I. For this reason, the lubricant oil can besufficiently retained there. Moreover, because the predetermined innerperipheral parts Iia can be easily machined in the inner peripheralsurface Ii of the input member I even using a general-purpose machinesuch as a lathe and the like, cost reduction can be achieved.

Meanwhile, FIGS. 3A and 3B show modified embodiments of the innerperipheral shape of the input member I. Specifically, in FIG. 3A, theinner peripheral surface Ii of the input member I is formed by joiningmultiple (two in the illustrated example) arcs with the same diameterwhose centers O″ are offset from the rotation axis L of the input memberI toward the corresponding pinions P on the cross-section orthogonal tothe rotation axis L of the input member I, while a central part of eacharc in the peripheral direction serves as the predetermined innerperipheral part Iia. The inner peripheral shape of the input member Imakes it possible to easily machine the predetermined inner peripheralparts Iia (the oil reservoirs) in the inner peripheral surface Ii of theinput member I even using the general-purpose machine such as the latheand the like. In addition, since the diameters of the multiple arcs areequal to one another, a machining tool such as a drill and the like forforming an arc surface can be used commonly for the arcs. For thesereasons, further cost reduction can be achieved.

Moreover, in FIG. 3B, the inner peripheral surface Ii of the inputmember I is formed in a shape of an ellipse whose major axis coincideswith an axis of the pinion shaft PS on the cross-section orthogonal tothe rotation axis L of the input member I, while each end portion of theellipse on the major axis side serves as the predetermined innerperipheral part Iia.

It should be noted that in addition to the embodiment shown in FIGS. 2,3A and 3B, various modified embodiments can be created for the innerperipheral shape of the input member I. For example, on thecross-section, the inner peripheral shape of the input member I may beformed in an oval shape (not illustrated) which is formed by joining apair of semicircles and a pair of short straight lines. In this case, acentral part of each semicircle in the peripheral direction serves asthe predetermined inner peripheral part Iia. Moreover, although in theembodiment, the predetermined inner peripheral parts Iia and the otherinner peripheral parts are smoothly connected to each other, steps maybe formed between the predetermined inner peripheral parts Iia and theother inner peripheral parts.

Next, descriptions will be provided for a structure for attaching thepinion shaft PS, as the pinion support portion, to the input member I.The pinion shaft PS has both end portions which are connected to andsupported by the input member I via attachment bodies T. A retaininghole Th is formed in each attachment body T (see FIG. 1), the retaininghole Th being able to be fitted therein with and retain an entireperiphery of the corresponding end portion of the pinion shaft PS.Furthermore, attachment grooves Ia each having a cross section with anangular U-shape are provided in the inner peripheral surface Ii of theinput member I, each of the attachment grooves Ia having an opening in aside surface of the input member I on the one cover portion C side andextending in the axial direction of the output shafts A. Each attachmentbody T having a rectangular parallelepiped shape is inserted into thecorresponding attachment groove Ia from the opening. The attachment bodyT is fixed to the input member I by fastening the one cover portion C tothe input member I using the bolts b, with the attachment body Tinserted in the attachment groove Ia of the input member I.

The above-described structure for attaching the pinion shaft PS to theinput member I enables the pinion shaft PS to be easily and firmlyconnected and fixed to the attachment grooves Ia in the input member Iby use of the block-shaped attachment bodies T in which the entireperipheries of the end portions of the pinion shaft PS are fitted andretained. For this reason, the pinion shaft PS can be connected to andsupported by the input member I with high strength, with no specializedthrough-hole for supporting the pinion shaft PS formed in the inputmember I, and without decreasing assembly workability. Furthermore, theembodiment achieves structure simplification since the cover portion Ccovering the outer side of the corresponding side gear S concurrentlyserves as the fixing means for retaining the attachment body T.

Thereby, when the both end portions of the pinion shaft PS are connectedto and supported by the input member I via the attachment bodies T,clearances 10 in the radial direction of the input member I are formedbetween outer end surfaces of the pinions P rotatably supported by thepinion shaft PS (i.e., end surfaces of the pinions P which are locatedoutward in the radial direction of the input member I) and the innerperipheral surface Ii of the input member I (i.e., the predeterminedinner peripheral parts Iia). This makes it easy for the lubricant oil tobe reserved in the clearances 10, and is accordingly effective toprevent seizure in end portions of the pinions P facing the clearances10, and their vicinities.

Meanwhile, the side wall portion Cs of the one cover portion C has astructure having oil retaining portions 7 covering parts of a backsurface of the side gear S in first predetermined areas including areaswhich overlap the pinions P as seen in a side view from outside in theaxial direction of the output shaft A (i.e., as seen in FIG. 2), andhaving lightening portions 8 exposing parts of the back surface of theside gear S to the outside of the differential case DC in secondpredetermined areas which do not overlap the pinions P as seen in theside view and connecting arm portions 9 being separated from the oilretaining portions 7 in the peripheral direction of the input member Iand extending in the radial direction of the input member I to connectthe boss portion Cb and the input member I. In other words, the sidewall portion Cs basically having a disk shape in the cover portion C hasa structural form in which: the multiple lightening portions 8 eachhaving a cutout shape are formed in the side wall portion Cs atintervals in the peripheral direction; and thereby, one oil retainingportion 7 and one connecting arm portion 9 are formed respectively onopposite sides of the lightening portion 8 in the peripheral direction.

Furthermore, in the embodiment, the lightening portions 8 are eachformed in a shape of a cutout which is opened on an outer peripheral endside of the side wall portion Cs and extends substantially along adirection orthogonal to the pinion shaft PS as seen in the side view.Thereby, the oil retaining portions 7 adjacent to the lighteningportions 8 are formed as long in the peripheral direction as possible.This enhances an oil retaining effect to be exhibited by the oilretaining portions 7, which will be described next.

The structural form of the side wall portion Cs of the cover portion C,particularly the oil retaining portions 7, makes it possible for thelubricant oil, which tends to move outward in the radial direction dueto the centrifugal force produced by the rotation of the input member I,to be easily retained around the pinions P and their vicinities. Forthis reason, in combination with the above-discussed oil concentratedreserving effect by the predetermined inner peripheral parts Iia (theoil reservoirs) of the input member I using the centrifugal force, it ispossible to more efficiently supply the lubricant oil to the pinions Pand their vicinities. Accordingly, even under severe driving conditionsand the like, such as the high-speed rotation of the pinions P, thelubricant oil can be more efficiently supplied to the sliding portionsof the pinions P, as well as the meshing portions of the pinions P andthe side gears S; and the seizure in the sliding portions and themeshing portions can be prevented more effectively.

In addition, since the cover portion C includes the lightening portions8, the lubricant oil can be distributed to the inside and outside of thedifferential case DC via the lightening portions 8. Thus, the lubricantoil is changed and cooled appropriately, thereby effectively preventingdegradation of the lubricant oil. Furthermore, since a large amount oflubricant oil need not be confined inside the differential case DC, andsince the cover portion C itself is reduced in weight by an amount ofthe forming of the lightening portions 8, reduction in the weight of thedifferential device D can be accordingly achieved.

It should be noted that although in the embodiment, the lighteningportions 8 are each formed in the cutout shape which is opened on theouter peripheral end side of the side wall portion Cs, the lighteningportions 8 may be instead each formed in a through-hole shape which isnot opened on the outer peripheral end side thereof. Furthermore, itshould be noted that although in the embodiment, the lightening portions8 are formed only in the side wall portion Cs of the one cover portion Cand the side wall portion Cs of the other cover portion C′ is formed ina disk shape with no lightening portion (accordingly, covering entirelythe back surfaces of the intermediate wall portion Sw and the toothingportion Sg of the corresponding side gear S), the lightening portions 8may be formed in the side wall portion Cs of the other cover portion C′as well. In this case, the oil retaining portions 7 and the connectingarm portions 9 are integrally formed in the input member I.

Meanwhile, like the connecting arm portions 9, the oil retainingportions 7 of the embodiment each extend between, and connect, the bossportion Cb of the cover portion C and the input member I. In addition,connecting of the cover portion C to the input member I by the oilretaining portions 7 makes it easier for the lubricant oil, which tendsto move outward in the radial direction due to the centrifugal forceduring the rotation of the input member I, to stay in spaces covered bythe oil retaining portions 7 and the input member I. This makes it easyfor the lubricant oil to be retained around the pinions P and theirvicinities.

It should be noted that the structure for connecting the oil retainingportions 7 and the connecting arm portions 9 to the input member I hasbeen described as the structure for connecting the cover portion C tothe input member I. In other words, the oil retaining portions 7 and theconnecting arm portions 9 may be formed integral with the input memberI. Otherwise, in a case where the oil retaining portions 7 and theconnecting arm portions 9 are formed separately from the input member I,the oil retaining portions 7 and the connecting arm portions 9 areconnected to the input member I using the screw means such as the boltsb and the like, or other various connecting means (for example, weldingmeans, swaging means and the like).

Furthermore, since the cover portion C has the structure in which thecover portion C integrally include the connecting arm portions 9 thatconnect the boss portion Cb and the input member I in addition to theoil retaining portions 7, the embodiment can accordingly increase:connecting strength with which the cover portion C is connected to theinput member I; rigidity strength of the cover portion C itself whichsupports the back surface of the corresponding side gear S; and supportrigidity with which the cover portion C supports the side gear S.Incidentally, the connecting arm portions 9 are not essential for thecover portion C, and another embodiment in which the connecting armportions 9 are removed from the cover portion C may be carried out.Furthermore, in a case where the cover portion C particularly includesthe connecting arm portions 9, another embodiment in which the oilretaining portions 7 are not connected to the input member I may becarried out.

Besides, the cover portion C of the embodiment has an oil guidinginclined surface f in a peripheral edge portion of each lighteningportion 8, the oil guiding inclined surface f being capable of guidingflow of the lubricant oil into an inner side of the input member Iduring the rotation of the input member I. As seen in a cross-sectioncrossing the oil retaining portions 7 and the connecting arm portions 9in the peripheral direction of the input member I (see the partiallycutaway sectional view in FIG. 2), the oil guiding inclined surface f isformed so as to be inclined to the respective center sides in theperipheral direction of the oil retaining portion 7 and the connectingarm portion 9, toward their respective inner side surfaces from theirrespective outer side surfaces. Thus, the oil guiding inclined surface fmakes it possible for the lubricant oil to smoothly flow from the outerside to the inner side of the cover portion C, and accordingly enhancesthe effect of lubricating the pinions P and the like.

Moreover, various modified embodiments can be created for the form ofthe lightening portions 8 (accordingly, the oil retaining portions 7 andthe connecting arm portions 9) of the cover portion C, and the form ofthe lightening portions 8 is not limited to the embodiment shown in FIG.2. For example, in a modified embodiment shown in FIG. 4, eachlightening portion 8 is formed in a shape of a fan whose center angle issubstantially 90 degrees in a way that the oil retaining portions 7 andthe connecting arm portions 9 extend radially (in other words, the oilretaining portions 7 and the connecting arm portions 9 have a crossshape as a whole).

Meanwhile, in each side gear S, at least part (in the embodiment, all)of the intermediate wall portion Sw is formed as a thin portion Swtwhose outer side surface retreats inward from the back surface of thetoothing portion Sg in the axial direction of the output shaft A (seeFIG. 1). On the other hand, each of the side wall portions Cs of thecover portions C, C′ (particularly, the oil retaining portions 7 and theconnecting arm portions 9 in the side wall portion Cs of the coverportion C) integrally includes: an outer periphery-side side wallportion Cso whose inner side surface faces the back surface of thetoothing portion Sg of the side gear S; and an inner periphery-side sidewall portion Csi whose inner side surface faces the back surface of theintermediate wall portion Sw of the side gear S. Furthermore, at leastpart (in the embodiment, all) of the inner periphery-side side wallportion Csi is formed thicker in a direction along the rotation axisthan the outer periphery-side side wall portion Cso, and protrudestoward the thin portion Swt.

Because of these structures, in each of the side gears S, at least partof the intermediate wall portion Sw, which does not need so muchrigidity as the toothing portion Sg, can be formed as the thin portionSwt retreating inward in the axial direction from the back surface ofthe toothing portion Sg; in each of the cover portions C, C′, each ofthe inner periphery-side side wall portions Csi of the cover portions C,C′ corresponding to the thin portion Swt can be made thicker withoutbeing protruded outward in the axial direction; and the support rigiditywith which the cover portion C supports the thin intermediate wallportion Sw of the side gear S can be sufficiently increased. This isextremely advantageous in sufficiently narrowing the width of thedifferential device D in the axial direction of the output shafts Awhile securing the rigidity strength of each side gear S and thedifferential case DC.

Moreover, as described above, the washers W relatively rotatablyconnecting the side gears S and the cover portions C, C′ are interposedbetween mutually-facing surfaces of the back surfaces of the side gearsS and the side wall portions Cs of the cover portions C, C′,respectively. In the embodiment, washer retaining grooves 6 forretaining the washers W at their fixed positions are formed in backsurfaces of the thin portions Swt of the side gears S, respectively.Thereby, the thin portions Swt with relatively low rigidity in the sidegears S can be directly supported by the washers W, and support strengthwith respect to the thin portions Swt can be increased. Furthermore,since the washers W are housed in and retained by the washer retaininggrooves 6, increase in the dimension of the differential device D in theaxial direction due to the thicknesses of the washers W can beinhibited.

Meanwhile, various modified embodiments can be created for a mode ofsetting the washers W to be interposed between the mutually-facingsurfaces of the back surfaces of the side gears S and the side wallportions Cs of the cover portions C, C′. For example, in FIG. 5A, thewasher retaining groove 6 is formed in an inner side surface of each ofthe cover portions C, C′ which faces the thin portion Swt of thecorresponding side gear S, and the washer W is retained by thethus-formed washer retaining groove 6. This avoids that the thin portionSwt is further thinned due to the washer retaining groove 6. Inaddition, in FIG. 5B, the washer retaining groove 6 is formed in theback surface of the toothing portion Sg of the side gear S, and thewasher W is retained by the thus-formed washer retaining groove 6. Thisshifts a load supporting point with respect to the side gear S furtheroutward in the radial direction (accordingly, to a position close to ameshing portion of the side gear S and the pinion P), and therebyincreases the supporting strength.

Furthermore, in FIG. 5C, a position of an inner periphery of the washerW is made to coincide with a start position at which the side wallportion Cs of each of the cover portions C, C′ starts to protrude inwardin the axial direction. Thereby, the mode of such inward protrusion ofthe side wall portion Cs is used to position the washer W. This makes itpossible to position and retain the washer W even though no washerretaining groove 6 is provided, and decrease in the strength due toforming of the washer retaining groove is avoided.

Moreover, in FIG. 5D, of the pinion shaft PS having a linear rod shapeextending in the radial direction (along the one diameter line) from therotation axis of the input member I, an intermediate shaft portion PSmfacing the thin portion Swt of each side gear S is formed with a smallerdiameter than that of another shaft portion of the pinion shaft PS.Thereby, the thin portion Swt is retreated and shifted inward in theaxial direction by the decrease in the diameter of the intermediateshaft portion PSm like this, and the side wall portion Cs (particularly,the inner periphery-side side wall portion Csi) of each of the coverportions C, C′ is made much thicker corresponding to such retreatingshift so as to increase the support rigidity with respect to thecorresponding side gear S.

Since as described above, each side gear S includes the intermediatewall portion Sw which is relatively wide in the radial direction, atorque transmission passage from the toothing portion Sg of side gear Sto the corresponding output shaft A becomes longer in the radialdirection so that the gear supporting strength may undesirably bedecreased. In the embodiment, however, the washer W can be properlydisposed and fixed at an appropriate radial position (see FIGS. 1 and 5Ato 5D) considered the gear supporting strength along the torquetransmission passage. For this reason, the embodiment can effectivelyinhibit the decrease in the gear supporting strength.

Next, descriptions will be provided for an operation of the embodiment.In the differential device D of the embodiment, in a case where theinput member I receives rotational force from a power source, whentogether with the input member I, the pinion P revolves around the axisL of the input member I without rotating around the pinion shaft PS, theleft and right side gears S are rotationally driven at the same speed,and their driving forces are evenly transmitted to the left and rightoutput shafts A. Meanwhile, when a difference in rotational speed occursbetween the left and right output shafts A due to turn traveling or thelike of the automobile, the pinion P revolves around the axis L of theinput member I while rotating around the pinion shaft PS. Thereby, therotational driving force is transmitted from the pinion P to the leftand right side gears S while allowing differential rotations. The aboveis the same as the operation of the conventional differential device.

Next, referring to FIGS. 6A to 6D, descriptions will be provided forsteps of manufacturing and assembling the differential device D of theembodiment. The steps include at least steps [1] to [6] as follows.

-   [1] A step of manufacturing and preparing a differential case main    body DC′, the cover portion C, the side gears S, the pinions P, the    pinion shaft PS, and the attachment bodies T, in their respective    separate steps, the differential case main body DC′ being obtained    by integrally forming the input member I and the cover portion C′    (or by connecting the input member I and the cover portion C′ which    are manufactured separately).-   [2] A step of fitting one side gear S into the differential case    main body DC′ as shown in FIG. 6A.-   [3] An assembly step of, as shown by solid lines in FIG. 6B,    assembling an attachment unit U such that the both end portions of    the pinion shaft PS are fitted and supported in center holes of the    pinions P and the retaining holes Th of the attachment bodies T, and    temporarily keeping the assembled state using a jig (not    illustrated).-   [4] A step of, as shown by arrows and chain double-dashed lines in    FIG. 6B, fitting the attachment unit U into the differential case    main body DC′ so as to insert the attachment bodies T into the    attachment grooves Ia of the input member I and so as to mesh the    pinions P with the toothing portion Sg of the one side gear S,    thereby detaching the attachment unit U from the jig, and therefore    temporarily fixing and retaining the attachment unit U to the input    member I.-   [5] A step of, as shown in FIG. 6C, overlapping the other side gear    S on an outside of the attachment unit U temporarily fixed and    retained to the input member I, and meshing the pinions P with the    toothing portion Sg of the other side gear S.-   [6] A step of, as shown in FIG. 6D, overlapping the cover portion C    on the back surface side of the other side gear S and fastening the    cover portion C to the input member I with the bolts b, thereby    clamping and fixing the attachment bodies T of the attachment unit U    between the cover portion C and inner surfaces of the attachment    grooves Ia of the input member I, thus completing the differential    device D.

In the series of steps, particularly in the assembly step [3], theattachment unit U is assembled and fixed to the input member I by:assembling the attachment unit U as a sub-assembly in advance, theattachment unit U being obtained by unitizing the pinion shaft PS, thepinions P and the attachment bodies T; thereafter positioning andretaining the attachment unit U in the input member I by inserting theattachment bodies T into the attachment grooves Ia of the input memberI; and thereafter fastening the cover portion C to the input member I.For this reason, assembly work efficiency can be effectively enhanced.

In addition, in the differential device D assembled as described above,each side gear S includes: the shaft portion Sj connected to the outputshaft A; and the intermediate wall portion Sw formed in a flat ringplate shape orthogonal to the axis L of the output shaft A, andintegrally connecting the shaft portion Sj and the side gear toothingportion Sg which is separated outward from the shaft portion Sj in theradial direction of the input member I. Furthermore, in each side gearS, the intermediate wall portion Sw is formed in the way that its widtht1 in the radial direction is longer than a maximum diameter d1 of eachpinion P. For these reasons, relative to the pinions P, the diameter ofthe side gear S can be made large enough to set the number Z1 of teethof the side gear S sufficiently larger than the number Z2 of teeth ofthe pinions P. This makes it possible to reduce load burden to thepinion shaft PS while the torque is being transmitted from the pinions Pto the side gears S, and thus to decrease the effective diameter d2 ofthe pinion shaft PS, accordingly the width of the pinions P in the axialdirection of the output shafts A.

In addition, since the load burden to the pinion shaft PS is reduced asdescribe above, since reaction force applied to each side gear Sdecreases, and since the back surface of the intermediate wall portionSw or the toothing portion Sg of the side gear S is supported by thecorresponding cover side wall portion Cs, it is easy to secure therigidity strength needed for the side gear S even though theintermediate wall portion Sw of the side gear S is thinned. That is tosay, it is possible to sufficiently thin the side gear intermediate wallportion Sw while securing the support rigidity with respect to the sidegear S. Moreover, in the embodiment, since the maximum thickness t2 ofthe side gear intermediate wall portion Sw is formed much smaller thanthe effective diameter d2 of the pinion shaft PS whose diameter can bemade smaller as described above, the further thinning of the side gearintermediate wall portion Sw can be achieved. Besides, since the coverside wall portion Cs is formed in a plate shape such that the outer sidesurface thereof is the flat surface orthogonal to the axis L of thecorresponding output shaft A, the thinning of the cover side wallportion Cs itself can be achieved.

As a result of these, the width of the differential device D can besufficiently decreased in the axial direction of the output shafts A asa whole while securing as approximately the same strength (for example,static torsion load strength) and as approximately the same amount ofmaximum torque transmission compared with the conventional differentialdevice. This makes it possible to easily incorporate the differentialdevice D, with great freedom and without trouble, even when atransmission system imposes many restrictions on the layout of thevicinity of the differential device D, and is extremely advantageous inreducing the size of the transmission system.

In addition, in the embodiment, it is desirable that the side gears Sand the pinions P be set to satisfy a relationship expressed withd3≧3.74·d2+20  (1)where d2 denotes the effective diameter of the pinion support portionPS, and d3 denotes a load point length of the pinions P (i.e., in anarea on and above a line X in FIG. 8A).

In this respect, the load point length d3 of the pinions P is twice aslong as the length of the distance from the rotational axis L to alarge-diameter end surface of one pinion P. For example, when the pairof pinions P are disposed facing each other, a distance between thelarge-diameter end surfaces of the pair of pinions P is the load pointlength d3 (see FIG. 1).

A line X1 shown in FIG. 8A represents a relationship between the pinionshaft diameter d2 and the load point length d3 of the pinions P in theconventional differential device. A predetermined static torsion loadstrength can be secured by setting the load point length d3 in a waythat a relationship of the load point length d3 with the pinion shaftdiameter d2 is represented by the line X1 . In contrast, in a settingexample in the embodiment, the line X whose gradient is equal to that ofthe line X1 and which makes the load point length d3 sufficiently largeis set; and the pinion shaft diameter d2 and the load point length d3 ofthe pinions P are set in the region on and above the line X. For thisreason, in the embodiment, the load point length of the pinions P can bemade sufficiently long, and the width of the differential device D canbe sufficiently decreased in the axial direction of the output shafts A,while securing the static torsion load strength which is not less thanthat of the conventional differential device.

Moreover, it is desirable that a pitch cone distance PCD of each pinionP as the bevel gear (i.e., a distance from a center of a fan shape ofthe pinion P having a longitudinal cross section with the fan shape toan outer end of the pinion P), the number Z2 of teeth of the pinion P,the number Z1 of teeth of each side gear S be set to satisfyrelationships expressed withZ1/Z2≧2  (2)PCD≧6.17·(Z1/Z2)+20  (3)(i.e., in an area in the right of and on a line Y, and on and above aline Z in FIG. 8B). To put it specifically, the line Y in FIG. 8Brepresents a gear ratio (Z1/ Z2) for sufficiently decreasing the widthof the differential device D in the axial direction of the output shaftsA. When the gear ratio (Z1/Z2) is set in the right of and on the line Y(i.e., when the gear ratio (Z1/Z2 ) is set at two or greater), theeffect of decreasing the width of the differential device D is large asshown in FIG. 8C. Meanwhile, in FIG. 8B, the line Z is a linerepresenting a relationship between the gear ratio and the pitch conedistance for obtaining an amount of torque transmission which isgenerally considered to be needed for four-wheeled automobiles, and isdetermined by plotting design values of the conventional differentialdevice. Accordingly, when the relationship between the gear ratio(Z1/Z2) and the pitch cone distance of the pinion P is set in a way thatthe relationship is included in the area in the right of and on the lineY, and on and above the line Z, the width of the differential device Dof the embodiment can be sufficiently decreased in the axial directionof the output shafts A (see FIG. 8C) while securing the amount ofmaximum torque transmission which is not less than that of theconventional differential device.

Meanwhile, although the embodiment where the long pinion shaft PS isused as the pinion support portion has been shown, the pinion supportportion may be formed from a support shaft portion PS′ coaxially andintegrally connected to a large diameter-side end surface of the pinionP as shown in FIG. 7. According to this configuration, because thethrough-hole into which the pinion shaft PS is fitted need not beprovided to the pinion P, the diameter of the pinion P can beaccordingly decreased (the width thereof can be decreased in the axialdirection), and the differential device D can be flattened in the axialdirection of the output shafts A. In other words, when the pinion shaftPS is penetrated through the pinion P, it is necessary to form in thepinion P the through-hole with a size corresponding to the pinion shaftdiameter. However, when the support shaft portion PS′ is integrated withthe end surface of the pinion P, it is possible to decrease the diameterof the pinion P (to decrease the width thereof in the axial direction)without depending on a diameter of the support shaft portion PS′.

Moreover, in the embodiment, a bearing bush 12 as a bearing for allowingrelative rotations between the support shaft portion PS′ and theattachment body T is inserted between an outer peripheral surface of thesupport shaft portion PS′ and an inner peripheral surface of theretaining hole Th of the corresponding attachment body T into which thesupport shaft portion PS′ is inserted. This bearing bush 12 is insertedbetween the inner periphery of the retaining hole Th of the attachmentbody T and the outer periphery of the support shaft portion PS′particularly in the assembly step [3]. This makes it possible toassemble the attachment unit U en masse, including the bearing bush 12,in the assembly step, and to therefore minimize the drop in the assemblywork efficiency even though the number of parts increases in response tothe addition of the bearing bush 12. Incidentally, the bearing may beformed from a needle bearing or the like. In addition, the bearing maybe omitted so that the support shaft portion PS′ is directly fitted intothe retaining hole Th of the attachment body T.

Meanwhile, in the conventional differential devices exemplified inJapanese Patent No. 4803871 and Japanese Patent Application KOKAIPublication No. 2002-364728 which are described above, the number Z1 ofteeth of the side gear (output gear) and the number Z2 of teeth of thepinion (differential gear) are generally set at 14 and 10, 16 and 10, or13 and 9, respectively, as shown in Japanese Patent Application KOKAIPublication No. 2002-364728, for example. In these cases, thenumber-of-teeth ratios Z1/Z2 of the output gears to the differentialgears are 1.4, 1.6 and 1.44, respectively. In addition, otherpublicly-known examples of the combination of the number Z1 of teeth andthe number Z2 of teeth for conventional differential devices include 15and 10, 17 and 10, 18 and 10, 19 and 10, and 20 and 10. In these cases,the number-of-teeth ratios Z1/Z2 are at 1.5, 1.7, 1.8, 1.9 and 2.0,respectively.

On the other hand, nowadays, there is an increase in the number oftransmission systems which are under layout restrictions around theirrespective differential devices. Accordingly, the market demands thatdifferential devices be sufficiently reduced in width (i.e., thinned) inthe axial direction of their output shafts while securing the gearstrength for the differential devices. However, the structural forms ofthe conventional existing differential devices are wide in the axialdirection of the output shafts, as is clear from the gear combinationsleading to the above-mentioned number-of-teeth ratios. This makes itdifficult to satisfy the market demand.

With this taken into consideration, an attempt to find a concreteconfiguration example of the differential device D which can besufficiently reduced in width (i.e., thinned) in the axial direction ofthe output shafts while securing the gear strength for the differentialdevice has been made as follows, from a viewpoint different from that ofthe foregoing embodiment. Incidentally, the structures of the componentsof the differential device D of this configuration example are the sameas the structures of the components of the differential device D of theforegoing embodiment which has been described using FIGS. 1 to 7. Forthis reason, the components of the configuration example will be denotedwith the same reference signs as those of the embodiment, anddescriptions for the structures will be omitted.

To begin with, let us explain a basic concept for sufficiently reducingthe width of (i.e., thinning) the differential device D in the axialdirection of the output shafts A referring to FIG. 10 together. Theconcept is as follows. Approach [1] To make the number-of-teeth ratioZ1/Z2 of the side gear S, that is, the output gear to the pinion P, thatis, the differential gear larger than the number-of-teeth ratio used forthe conventional existing differential device. (This leads to a decreasein the module (accordingly the tooth thickness) of the gear and aresultant decrease in the gear strength, while leading to an increase inthe pitch circle diameter of the side gear S, a resultant decrease intransmission load in the meshing portion of the gear, and a resultantincrease in the gear strength. However, the gear strength as a wholedecreases, as discussed below.)

Approach [2] To make the pitch cone distance PCD of the pinion P largerthan the pitch cone distance in the conventional existing differentialdevice. (This leads to an increase in the module of the gear and aresultant increase in the gear strength, while leading to an increase inthe pitch circle diameter of the side gear S, a resultant decrease inthe transmission load in the meshing portion of the gear, and aresultant increase in the gear strength. Thus, the gear strength as awhole increases greatly, as discussed below.)

For these reasons, when the number-of-teeth ratio Z1/Z2 and the pitchcone distance PCD are set such that the amount of decrease in the gearstrength based on Approach [1] is equal to the amount of increase in thegear strength based on Approach [2] or such that the amount of increasein the gear strength based on Approach [2] is greater than the amount ofdecrease in the gear strength based on Approach [1], the gear strengthas a whole can be made equal to or greater than that of the conventionalexisting differential device.

Next, let us concretely examine how the gear strength changes based onApproaches [1] and [2] using mathematical expressions. Incidentally, theexamination will be described in the following embodiment. First of all,a “reference differential device” is defined as a differential device D′in which the number Z1 of teeth of the side gear S is set at 14 whilethe number Z2 of teeth of the pinion P is set at 10. In addition, foreach variable, a “change rate” is defined as a rate of change in thevariable in comparison with the corresponding base number (i.e., 100%)of the reference differential device D′.

Approach [1]

When M, PD₁, θ₁, PCD, F, and T respectively denote the module, pitchcircle diameter, pitch angle, pitch cone distance, transmission load inthe gear meshing portion, and transmission torque in the gear meshingportion, of the side gear S, general formulae concerning the bevel gearprovideM=PD ₁ /Z1,PD ₁=2PCD·sin θ ₁, andθ₁ =tan ⁻¹(Z1/Z2).From these expressions, the module of the gear is expressed withM=2PCD·sin { tan ⁻¹(Z1/Z2)}/Z1  (1)

Meanwhile, the module of the reference differential device D′ isexpressed with2PCD·sin { tan ⁻¹(7/5)}/14.

Dividing the term on the right side of Expression (1) by 2PCD·sin {tan⁻¹(7/5)}/14 yields a module change rate with respect to the referencedifferential device D′, which is expressed with Expression (2) givenbelow.

$\begin{matrix}{{{Module}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = \frac{14 \cdot {\sin\left( {\tan^{- 1}\frac{Z1}{Z\; 2}} \right)}}{Z\;{1 \cdot {\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} & (2)\end{matrix}$

In addition, the section modulus of the tooth portion corresponding tothe gear strength (i.e., the bending strength of the tooth portion) isin proportion to the square of the tooth thickness, while the tooththickness has a substantially linear relationship with the module M. Forthese reasons, the square of the module change rate corresponds to arate of change in the section modulus of the tooth portion, accordinglya gear strength change rate. In other words, based on Expression (2)given above, the gear strength change rate is expressed with Expression(3) given below. Expression (3) is represented by a line L1 in FIG. 11when the number Z2 of teeth of the Pinion is 10. From the line L1, it islearned that as the number-of-teeth ratio Z1/Z2 becomes larger, themodule becomes smaller and the gear strength accordingly becomes lower.

$\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = {\left( {{Module}\mspace{14mu}{Change}\mspace{14mu}{Rate}} \right)^{2} = \frac{196 \cdot {\sin^{2}\left( {\tan^{- 1}\frac{Z\; 1}{Z2}} \right)}}{Z\;{1^{2} \cdot {\sin^{2}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}}} & (3)\end{matrix}$

Meanwhile, based on the general formulae concerning the bevel gear, atorque transmission distance of the side gear S is expressed withExpression (4) given below.PD ₁/2=PCD·sin { tan ⁻¹(Z1/Z2)}  (4)

From the torque transmission distance PD₁/2, the transmission load F isgiven asF=2T/PD ₁.For this reason, when the torque T of the side gear S of the referencedifferential device D′ is constant, the transmission load F is ininverse proportion to the pitch circle diameter PD₁. In addition, therate of change in the transmission load F is in inverse proportion tothe gear strength change rate. For this reason, the gear strength changerate is equal to the rate of change in the pitch circle diameter PD₁.

As a result, using Expression (4), the rate of change in the pitchcircle diameter PD₁ is expressed with Expression (5) given below.

$\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = {{{PD}_{1}\mspace{14mu}{Change}\mspace{14mu}{Rate}} = \frac{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}}} & (5)\end{matrix}$

Expression (5) is represented by a line L2 in FIG. 11 when the number Z2of teeth of the pinion P is 10. From the line L2, it is learned that asthe number-of-teeth ratio Z1/Z2 becomes larger, the transmission loadbecomes smaller, and the gear strength accordingly becomes stronger.

Eventually, the gear strength change rate in accordance with theincrease in the number-of-teeth ratio Z1/Z2 is expressed with Expression(6) given below by multiplying a rate of decrease change in the gearstrength in accordance with the decrease in the module M (the term onthe right side of Expression (3) shown above) and a rate of increasechange in the gear strength in accordance with the decrease in thetransmission load (the term on the right side of Expression (5) shownabove).

$\begin{matrix}{{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}\mspace{14mu}{Rate}\mspace{14mu}{in}\mspace{14mu}{Accordance}\mspace{11mu}{with}\mspace{14mu}{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu}{Ratio}} = \frac{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\;{1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} & (6)\end{matrix}$

Expression (6) is represented by a line L3 in FIG. 11 when the number Z2of teeth of the pinion P is 10. From the line L3, it is learned that asthe number-of-teeth ratio Z1/Z2 becomes larger, the gear strength as awhole becomes lower.

Approach [2]

In a case of increasing the pitch cone distance PCD of the pinion P morethan the pitch cone distance in the reference differential device D′,when PCD1, PCD2 respectively denote the pitch cone distance PCD beforethe change and the pitch cone distance PCD after the change, the modulechange rate in accordance with the change in the pitch cone distance PCDis expressed withPCD2/PCD1if the number of teeth is constant, based on the above-mentioned generalformulae concerning the bevel gear.

Meanwhile, as being clear from the above-discussed process for derivingExpression (3), the gear strength change rate of the side gear Scorresponds to the square of the module change rate. For this reason,Gear Strength Change Rage in Accordance with Increase inModule=(PCD2/PCD1)²  (7)is obtained. Expression (7) is represented by a line L4 in FIG. 12. Fromthe line L4, it is learned that as the pitch cone distance PCD becomeslarger, the module becomes larger, and the gear strength accordinglybecomes stronger.

In addition, when the pitch cone distance PCD is made larger than thepitch cone distance PCD1 in the reference differential device D′, thetransmission load F decreases. Thereby, the gear strength change ratebecomes equal to the rate of change in the pitch circle diameter PD₁, asdescribed above. In addition, the pitch circle diameter PD₁ of the sidegear S is in proportion to the pitch cone distance PCD. For thesereasons,Gear Strength Change Rate in Accordance with Decrease in TransmissionLoad =PCD2/PCD1  (8)is obtained.

Expression (8) is represented by a line L5 in FIG. 12. From the line L5,it is learned that as the pitch cone distance PCD becomes larger, thetransmission load becomes lower, and the gear strength accordinglybecomes stronger.

In addition, the gear strength change rate in accordance with theincrease in the pitch cone distance PCD is expressed with Expression (9)given below by multiplying the rate of increase change in the gearstrength in accordance with the increase in the module M (the term onthe right side of Expression (7) shown above) and the rate of increasechange in the gear strength in accordance with the decrease in thetransmission load in response to the increase in the pitch circlediameter PD (the term on the right side of Expression (8) shown above).Gear Strength Change Rate in Accordance with Increase in Pitch ConeDistance=(PCD2/PCD1)³  (9)

Expression (9) is represented by a line L6 in FIG. 12. From the line 6,it is learned that as the pitch cone distance PCD becomes larger, thegear strength is increased greatly.

With these taken into consideration, the combination of thenumber-of-teeth ratio Z1/Z2 and the pitch cone distance PCD isdetermined such that: the decrease in the gear strength based onApproach [1] given above (the increase in the number-of-teeth ratio) issufficiently compensated for by the increase in the gear strength basedon Approach [2] given above (the increase in the pitch cone distance) soas to make the overall gear strength of the differential device equal toor greater than the gear strength of the conventional existingdifferential device.

For example, 100% of the gear strength of the side gear S of thereference differential device D′ can be kept by setting the gearstrength change rate in accordance with the increase in thenumber-of-teeth ratio (i.e., the term on the right side of Expression(6) given above) obtained based on Approach [1] given above and the gearstrength change rate in accordance with the increase in the pitch conedistance (i.e., the term on the right side of Expression (9) givenabove) obtained based on Approach [2] given above, such that themultiplication of these gear strength change rates becomes equal to100%. Thereby, the relationship between the number-of-teeth ratio Z1/Z2and the rate of change in the pitch cone distance PCD for keeping 100%of the gear strength of the reference differential device D′ can beobtained from Expression (10) given below. Expression (10) isrepresented by a line L7 in FIG. 13 when the number Z2 of teeth of thepinion P is 10.

$\begin{matrix}\begin{matrix}{{{PCD}\;{2/{PCD}}\; 1} = \left( {100{\%/\begin{matrix}{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}} \\{{Rate}\mspace{14mu}{in}\mspace{14mu}{Accordance}\mspace{14mu}{with}} \\{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu}{Ratio}}\end{matrix}}} \right)^{\frac{1}{3}}} \\{= \left\{ \frac{1}{\frac{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\;{1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} \right\}^{\frac{1}{3}}} \\{= {\left( \frac{\;{Z\; 1}}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix} & (10)\end{matrix}$

Like this, Expression (10) represents the relationship between thenumber-of-teeth ratio Z1/Z2 and the rate of change in the pitch conedistance PCD for keeping 100% of the gear strength of the referencedifferential device D′ when the number-of-teeth ratio Z1/Z2 is equal to14/10 (see FIG. 13). The rate of change in the pitch cone distance PCDrepresented by the vertical axis in FIG. 13 can be converted into aratio of d2/PCD where d2 denotes a shaft diameter of the pinion shaft PS(i.e., the differential gear support portion) supporting the pinion P.

TABLE 1 SHAFT PCD DIAMETER (d2) d2/PCD 31 13 42% 35 15 43% 38 17 45% 3917 44% 41 18 44% 45 18 40%

To put it concretely, in the conventional existing differential device,the increase change in the pitch cone distance PCD correlates with theincrease change in the shaft diameter d2 as shown in Table 1, and can berepresented by a decrease in the ratio of d2 /PCD when d2 is constant.In addition, in the conventional existing differential device, d2/PCDfalls within a range of 40% to 45% as shown in Table 1 given above whenthe conventional existing differential device is the referencedifferential device D′, and the gear strength increases as the pitchcone distance PCD increases. Judging from these, the gear strength ofthe differential device can be made equal to or greater than the gearstrength of the conventional existing differential device by determiningthe shaft diameter d2 of the pinion shaft PS and the pitch cone distancePCD such that at least d2/PCD is equal to or less than 45%, when thedifferential device is the reference differential device D′. In otherwords, when the differential device is the reference differential deviceD′, it suffices if d2/PCD≦0.45 is satisfied. In this case, when PCD2denotes the pitch cone distance PCD which is changed to become larger orless than the pitch cone distance PCD1 of the reference differentialdevice D′, it suffices ifd2/PCD2≦0.45/(PCD2/PCD1)  (11)is satisfied. Furthermore, the application of Expression (11) toExpression (10) given above can convert the relationship between d2/PCDand the number-of-teeth ratio Z1/Z2 into Expression (12) given below.

$\begin{matrix}\begin{matrix}{{d\;{2/{PCD}}}\; \leqq {0.45/\left( {{PCD}\;{2/{PCD}}\; 1} \right)}} \\{= {0.45/\left\{ {\left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}} \right\}}} \\{= {0.45 \cdot \left( \frac{14}{Z\; 1} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}}}\end{matrix} & (12)\end{matrix}$

When the Expression (12) is equal, Expression (12) can be represented bya line L8 in FIG. 14 if the number Z2 of teeth of the pinion P is 10.When the Expression (12) is equal, the relationship between d2/PCD andthe number-of-teeth ratio Z1/Z2 keeps 100% of the gear strength of thereference differential device D′.

Meanwhile, in conventional existing differential devices, usually, notonly the number-of-teeth ratio Z1/Z2 equal to 1.4 used above to explainthe reference differential device D′ but also the number-of-teeth ratioZ1/Z2 equal to 1.6 or 1.44 is adopted. This needs to be taken intoconsideration. Based on the assumption that the reference differentialdevice D′ (Z1/Z2=1.4) guarantee the necessary and sufficient gearstrength, that is, 100% of gear strength, it is learned, as being clearfrom FIG. 11, that the gear strength of conventional existingdifferential devices in which the number-of-teeth ratio Z1/Z2 is 16/10is as low as 87% of the gear strength of the reference differentialdevice D′. The general practice, however, is that the gear strength lowat that level is accepted as practical strength and actually used forconventional existing differential devices. Judging from this, one mayconsider that gear strength which needs to be sufficiently secured forand is acceptable for the differential device which is thinned in theaxial direction is at least equal to, or greater than, 87% of the gearstrength of the reference differential device D′.

From the above viewpoint, first, a relationship for keeping 87% of thegear strength of the reference differential device D′ is obtainedbetween the number-of-teeth ratio Z1/Z2 and the rate of change in thepitch cone distance PCD. The relationship can be expressed withExpression (10′) given below by performing a calculation by emulatingthe process of deriving Expression (10) given above (i.e., a calculationsuch that the multiplication of the gear strength change rate inaccordance with the increase in the number-of-teeth ratio (i.e., theterm on the right side of Expression (6) given above) and the gearstrength change rate in accordance with the increase in the pitch conedistance (i.e., the term on the right side of Expression (9) givenabove) becomes equal to 87%).

$\begin{matrix}\begin{matrix}{{{PCD}\;{2/{PCD}}\; 1} = \left( {87{\%/\begin{matrix}{{Gear}\mspace{14mu}{Strength}\mspace{14mu}{Change}} \\{{Rate}\mspace{14mu}{in}\mspace{14mu}{Accordance}\mspace{14mu}{with}} \\{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu}{Ratio}}\end{matrix}}} \right)^{\frac{1}{3}}} \\{= \left\{ \frac{0.87}{\frac{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\;{1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} \right\}^{\frac{1}{3}}} \\{= {0.87^{\frac{1}{3}} \cdot \left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix} & \left( 10^{\prime} \right)\end{matrix}$

Thereafter, when Expression (11) given above is applied to Expression(10′) given above, the relationship between d2/PCD and thenumber-of-teeth ratio Z1/Z2 for keeping 87% or more of the gear strengthof the reference differential device D′ can be converted into Expression(13) given below. However, the calculation is performed using thefollowing rules that: the number of significant figures is three for allthe factors, except for factors expressed with variables; digits belowthe third significant figure are rounded down; and although the resultof the calculation cannot avoid approximation by an calculation error,the mathematical expression uses the equals sign because the error isnegligible.

$\begin{matrix}\begin{matrix}{{d\;{2/{PCD}}}\; \leqq {0.45/\left\{ {0.87^{\frac{1}{3}} \cdot \left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin\left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}} \right\}}} \\{= {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix} & (13)\end{matrix}$

When the Expression (13) is equal, Expression (13) can be represented byFIG. 14 (more specifically, by a line L9 in FIG. 14) if the number Z2 ofteeth of the pinion P is 10. In this case, an area corresponding toExpression (13) is an area on and under the line L9 in FIG. 14. Inaddition, a specific area (a hatched area in FIG. 14) satisfyingExpression (13) and located on the right side of a line L10 in FIG. 14where the number-of-teeth ratio Z1/Z2 >2.0 is satisfied is an area forsetting Z1/Z2 and d2/PCD which enable at least 87% or more of the gearstrength of the reference differential device D′ to be securedparticularly for the differential device thinned in the axial directionwhere the number Z2 of teeth of the pinion P is 10 and thenumber-of-teeth ratio Z1/Z2 is greater than 2.0. For reference, a blackdiamond in FIG. 14 represents an example where the number-of-teeth ratioZ1/Z2 and d2/PCD are set at 40/10 and 20.00%, respectively, and a blacktriangle in FIG. 14 represents an example where the number-of-teethratio Z1/Z2 and d2/PCD are set at 58/10 and 16.67%, respectively. Theseexamples fall within the specific area. A result of a simulation forstrength analysis on these examples has confirmed that the gear strengthequal to or greater than those of the conventional differential devices(more specifically, the gear strength equal to or greater than 87% ofthe gear strength of the reference differential device D′) wereobtained.

Thus, the thinned differential device falling within the specific areais configured as the differential device which, as a whole, issufficiently reduced in width in the axial direction of the outputshafts while securing the gear strength (for example, static torsionload strength) and the maximum amount of torque transmission atapproximately the same levels as the conventional existing differentialdevices which are not thinned in the axial direction thereof.Accordingly, it is possible to achieve effects of: being capable ofeasily incorporating the differential device in a transmission system,which is under many layout restrictions around the differential device,with great freedom and no specific difficulties; being extremelyadvantageous in reducing the size of the transmission system; and thelike.

It should be noted that although the foregoing descriptions (thedescriptions in connection with FIGS. 11, 13, 14 in particular) havebeen provided for the differential device in which the number Z2 ofteeth of the pinion P is set at 10, the present invention is not limitedto this. For example, when the number Z2 of teeth of the pinion P is setat 6, 12 and 20, too, the thinned differential device capable ofachieving the above effects can be represented by Expression (13), asshown by hatched areas in FIGS. 15, 16 and 17. In other words,Expression (13) derived in the above-described manner is applicableregardless of the change in the number Z2 of teeth of the pinion P. Forexample, even when the number Z2 of teeth of the pinion P is set at 6,12 and 20, the above effects can be obtained by setting the number Z1 ofteeth of the side gear S, the number Z2 of teeth of the pinion P, theshaft diameter d2 of the pinion shaft PS and the pitch cone distance PCDsuch that Expression (13) is satisfied, like in the case where thenumber Z2 of teeth of the pinion P is set at 10.

Furthermore, for reference, a black diamond in FIG. 16 represents anexample where when the number Z2 of teeth of the pinion P is 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 48/12 and 20.00%,respectively, and a black triangle in FIG. 16 represents an examplewhere when the number Z2 of teeth of the pinion P is 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 70/12 and 16.67%,respectively. A result of a simulation for strength analysis on theseexamples has confirmed that the gear strength equal to or greater thanthose of the conventional differential devices (more specifically, thegear strength equal to or greater than 87% of the gear strength of thereference differential device D′) were obtained. Moreover, theseexamples fall within the specific area, as shown in FIG. 16.

As comparative examples, let us show examples which do not fall withinthe specific area. A white star in FIG. 14 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 10, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 58/10 and 27.50%,respectively, and a white circle in FIG. 14 represents an example wherewhen the number Z2of teeth of the pinion P is for example 10, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 40/10 and 34.29%,respectively. A white star in FIG. 16 represents an example where whenthe number Z2 of teeth of the pinion P is for example 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 70/12 and 27.50%,respectively, and a white circle in FIG. 16 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 48/12 and 34.29%,respectively. A result of a simulation for strength analysis on theseexamples has confirmed that the gear strength equal to or greater thanthose of the conventional differential devices (more specifically, thegear strength equal to or greater than 87% of the gear strength of thereference differential device D′) were not obtained. In other words, theabove effects cannot be obtained from the examples which do not fallwithin the specific area.

Although the embodiment of the present invention has been described, thepresent invention is not limited to the embodiment. Various designchanges may be made to the present invention within a scope notdeparting from the gist of the present invention.

For example, in any of the embodiments, in each side gear S, the thinportion Swt is formed by making the back surface side of theintermediate wall portion Sw retreat inward in the axial direction ofthe output shaft A. For example, however, in each side gear S, the backsurface side of the intermediate wall portion Sw is formed as a flatsurface flush with the back surface of the toothing portion Sg withoutmaking the back surface side of the intermediate wall portion Sw retreatinward in the axial direction, as shown in FIG. 9. In this case, theentirety of the inner side surface of the side wall portion Cs of eachof the cover portions C, C′ is also formed as a flat surface in whichthe inner periphery-side side wall portion Cso and the outerperiphery-side side wall portion Csi are flush with each other.

In addition, although the embodiment where the lightening portions 8 areprovided to the side wall portion Cs of at least the one cover portion Cof the left and right cover portions C, C′ has been shown, thelightening portions 8 may be formed in the side wall portions Cs ofneither of the left and right cover portions C, C′ so that each sidewall portion Cs covers the entirety of the back surface of thecorresponding side gear.

Furthermore, although the embodiment where the input member I integrallyincludes the input toothing portion Ig has been shown, a ring gear whichis formed separately from the input member I may be fixed to the inputmember I later instead of the input toothing portion Ig. Moreover, theinput member of the present invention may have a structure whichincludes neither the input toothing portion Ig nor the ring gear. Forexample, the input member I may be operatively connected to a drivemember (for example, an output member of a planetary gear mechanism or areduction gear mechanism, a driven wheel of an endless transmissionbelt-type transmission mechanism and the like) situated upstream of theinput member I on the power transmission passage so that the rotationaldriving force is inputted into the input member I.

Moreover, the embodiment where the back surfaces of the pair of sidegears S are covered with the pair of cover portions C, C′ has beenshown, however, in the present invention, the back surface of only oneside gear S may be provided with the cover portion. In this case, forexample, the upstream-situated drive member may be disposed on the sidegear side provided with no cover portion so that the drive member andthe input member are operatively connected to each together on the sidegear side provided with no cover portion.

What is claimed is:
 1. A differential device which distributivelytransmits rotational force of an input member to a pair of outputshafts, the input member retaining a pinion support portion thatsupports a pinion and being rotatable together with the pinion supportportion, the differential device comprising: a pair of side gears eachhaving an annular toothing portion in mesh with the pinion in an outerperipheral portion and connected to the pair of output shafts,respectively; and a cover portion axially covering an outside of atleast one side gear of the side gears and rotating integrally with theinput member, wherein the pair of side gears include shaft portionsconnected to the pair of output shafts, respectively, and intermediatewall portions formed in a flat plate shape intersecting an axis of theoutput shafts, the intermediate wall portions integrally connecting theshaft portions to the annular toothing portions separated outward fromthe shaft portions in a radial direction of the input member,respectively, each intermediate wall portion is formed with a width inthe radial direction which is greater than a maximum diameter of thepinion, the cover portion includes a boss portion concentricallysurrounding the corresponding shaft portion, and a plate-shaped sidewall portion connected to the boss portion and having an outer sidesurface as a surface orthogonal to the axis of the output shafts,wherein: a back surface of at least one of the intermediate wallportions and the annular toothing portion of the side gear is rotatablysupported on an inner side surface of a corresponding one of the sidewall portions of the cover portion, the input member includes an inputtoothing portion having a plurality of teeth extending in a directionsubstantially parallel to an axial direction of the output shafts, theinput toothing portion configured to receive rotational force from apower source in an outer peripheral portion of the input member, and theside wall portion of the cover portion is disposed within a width of theinput toothing portion in the axial direction of the output shafts. 2.The differential device according to claim 1, wherein the intermediatewall portion of at least one side gear of the pair of side gears isformed with a maximum thickness in an axial direction of the outputshafts which is smaller than an effective diameter of the pinion supportportion.
 3. The differential device according to claim 2, wherein arelationship expressed by d3≧3.74·d2+20 is satisfied, where d2 denotesan effective diameter of the pinion support portion and d3 denotes aload point length of the pinion.
 4. The differential device according toclaim 3, wherein the pinion and the toothing portion of each of the sidegears are bevel gears, and relationships expressed by Z1/Z2≧2, andPCD≧6.17·(Z1/Z2)+20 are satisfied, where PCD denotes a pitch conedistance of the pinion, Z2 denotes the number of teeth of the pinion andZ1 denotes the number of teeth of the side gear.
 5. The differentialdevice according to claim 2, wherein the pinion and the toothing portionof each of the side gears are bevel gears, and relationships expressedby Z1/Z2≧2, and PCD≧6.17·(Z1/Z2)+20 are satisfied, where PCD denotes apitch cone distance of the pinion, Z2 denotes the number of teeth of thepinion and Z1 denotes the number of teeth of the side gear.
 6. Thedifferential device according to claim 1, wherein a relationshipexpressed by d3≧3.74·d2+20 is satisfied, where d2 denotes an effectivediameter of the pinion support portion and d3 denotes a load pointlength of the pinion.
 7. The differential device according to claim 6,wherein the pinion and the toothing portion of each of the side gearsare bevel gears, and relationships expressed by Z1/Z2≧2, andPCD≧6.17·(Z1/Z2)+20 are satisfied, where PCD denotes a pitch conedistance of the pinion, Z2 denotes the number of teeth of the pinion andZ1 denotes the number of teeth of the side gear.
 8. The differentialdevice according to claim 1, wherein the pinion and the toothing portionof each of the side gears are bevel gears, and relationships expressedby Z1/Z2≧2, and PCD≧6.17·(Z1/Z2)+20 are satisfied, where PCD denotes apitch cone distance of the pinion, Z2 denotes the number of teeth of thepinion and Z1 denotes the number of teeth of the side gear.
 9. Thedifferential device according to claim 1, wherein the cover portioncomprises a first cover portion axially covering the outside of the atleast one side gear of the side gears, and a second cover portionaxially covering the outside of the other side gear of the side gears,and wherein the side wall portions of the first and second coverportions are disposed within the width of the input toothing portion inthe axial direction of the output shafts, respectively.
 10. Adifferential device comprising: an input member into which driving forceis input; a differential gear support portion supported in the inputmember; a differential gear supported on the differential gear supportportion; a pair of output gears in mesh with the differential gear andrelatively rotatable to the differential gear; and a cover portionaxially covering an outside of at least one output gear of the outputgears and rotating integrally with the input member, wherein: each ofthe output gears includes a shaft portion connected to a correspondingone of a pair of output shafts, and a toothing portion in mesh with thedifferential gear, the cover portion includes a boss portion surroundingthe shaft portion of the associated output gear, and a side wall portionconnected to the boss portion, the input member has an input toothingportion having a plurality of teeth extending in a directionsubstantially parallel to an axial direction of the output shafts, theinput toothing portion configured to receive the drive force in an outerperipheral portion of the input member, and the side wall portion of thecover portion is disposed within a width of the input toothing portionin the axial direction of the output shafts, and further wherein${d\;{2/{PCD}}} \leqq {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}$is satisfied, and Z1/Z2>2 is satisfied, where Z1, Z2, d2 and PCD denotethe number of teeth of each of the output gears, the number of teeth ofthe differential gear, a diameter of the differential gear supportportion and a pitch cone distance, respectively.
 11. The differentialdevice according to claim 10, wherein Z1/Z2≧4 is satisfied.
 12. Thedifferential device according to claim 10, wherein Z1/Z2≧5.8 issatisfied.
 13. The differential device according to claim 10, whereinthe cover portion comprises a first cover portion axially covering theoutside of the at least one output gear of the output gears and a secondcover portion axially covering the outside of the other output gear ofthe output gears, and wherein the side wall portions of the first andsecond cover portions are disposed within the width of the input memberin the axial direction of the output shafts, respectively.